Not just the koran, Almost all religions claim these stupid things. It's because people of those areas hadn't the slightest clue how stuff worked and so they made up their own fantasies and spread them all over the world, Popularising their nonsense. Even the ancient Greeks knew the earth wasn't in fact flat. Religion really Hinders scientific development. Tsh.
The Koran says. . .
Sura Taha (20:53)
Who has made earth for you like a bed (spread out); and has opened roads (ways and paths etc. ) for you therein; and has sent down water (rain) from the sky. And We have brought forth with it various kinds of vegetation.
Sura Az-Zukhruf (43:10)
Who has made for you the earth like a bed, And has made for you roads therein, In order that you may find your way.
Sura Al-Kahf (18:86)
Until, When he reached the setting place of the sun, He found it setting in a spring of black muddy (or hot) water. And he found near it a people. We (Allâh) said (by inspiration): "O Dhul-Qarnain! Either you punish them, Or treat them with kindness. "